linux-kernel - Re: [PATCH v2 04/12] mm: Assign memcg-aware shrinkers bitmap to memcg

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Date:   Sat, 28 Apr 2018 18:08:27 +0300
From:   Vladimir Davydov <vdavydov.dev@...il.com>
To:     Kirill Tkhai <ktkhai@...tuozzo.com>
Cc:     akpm@...ux-foundation.org, shakeelb@...gle.com,
        viro@...iv.linux.org.uk, hannes@...xchg.org, mhocko@...nel.org,
        tglx@...utronix.de, pombredanne@...b.com, stummala@...eaurora.org,
        gregkh@...uxfoundation.org, sfr@...b.auug.org.au, guro@...com,
        mka@...omium.org, penguin-kernel@...ove.SAKURA.ne.jp,
        chris@...is-wilson.co.uk, longman@...hat.com, minchan@...nel.org,
        hillf.zj@...baba-inc.com, ying.huang@...el.com,
        mgorman@...hsingularity.net, jbacik@...com, linux@...ck-us.net,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        willy@...radead.org, lirongqing@...du.com, aryabinin@...tuozzo.com
Subject: Re: [PATCH v2 04/12] mm: Assign memcg-aware shrinkers bitmap to memcg

On Tue, Apr 24, 2018 at 03:24:53PM +0300, Kirill Tkhai wrote:
> >>>>>> +int expand_shrinker_maps(int old_nr, int nr)
> >>>>>> +{
> >>>>>> +	int id, size, old_size, node, ret;
> >>>>>> +	struct mem_cgroup *memcg;
> >>>>>> +
> >>>>>> +	old_size = old_nr / BITS_PER_BYTE;
> >>>>>> +	size = nr / BITS_PER_BYTE;
> >>>>>> +
> >>>>>> +	down_write(&shrinkers_max_nr_rwsem);
> >>>>>> +	for_each_node(node) {
> >>>>>
> >>>>> Iterating over cgroups first, numa nodes second seems like a better idea
> >>>>> to me. I think you should fold for_each_node in memcg_expand_maps.
> >>>>>
> >>>>>> +		idr_for_each_entry(&mem_cgroup_idr, memcg, id) {
> >>>>>
> >>>>> Iterating over mem_cgroup_idr looks strange. Why don't you use
> >>>>> for_each_mem_cgroup?
> >>>>
> >>>> We want to allocate shrinkers maps in mem_cgroup_css_alloc(), since
> >>>> mem_cgroup_css_online() mustn't fail (it's a requirement of currently
> >>>> existing design of memcg_cgroup::id).
> >>>>
> >>>> A new memcg is added to parent's list between two of these calls:
> >>>>
> >>>> css_create()
> >>>>   ss->css_alloc()
> >>>>   list_add_tail_rcu(&css->sibling, &parent_css->children)
> >>>>   ss->css_online()
> >>>>
> >>>> for_each_mem_cgroup() does not see allocated, but not linked children.
> >>>
> >>> Why don't we move shrinker map allocation to css_online then?
> >>
> >> Because the design of memcg_cgroup::id prohibits mem_cgroup_css_online() to fail.
> >> This function can't fail.
> > 
> > I fail to understand why it is so. Could you please elaborate?
> 
> mem_cgroup::id is freed not in mem_cgroup_css_free(), but earlier. It's freed
> between mem_cgroup_css_offline() and mem_cgroup_free(), after the last reference
> is put.
> 
> In case of sometimes we want to free it in mem_cgroup_css_free(), this will
> introduce assymmetric in the logic, which makes it more difficult. There is
> already a bug, which I fixed in
> 
> "memcg: remove memcg_cgroup::id from IDR on mem_cgroup_css_alloc() failure"
> 
> new change will make this code completely not-modular and unreadable.

How is that? AFAIU all we need to do to handle css_online failure
properly is call mem_cgroup_id_remove() from mem_cgroup_css_free().
That's it, as mem_cgroup_id_remove() is already safe to call more
than once for the same cgroup - the first call will free the id
while all subsequent calls will do nothing.

>  
> >>
> >> I don't think it will be good to dive into reworking of this stuff for this patchset,
> >> which is really already big. Also, it will be assymmetric to allocate one part of
> >> data in css_alloc(), while another data in css_free(). This breaks cgroup design,
> >> which specially introduces this two function to differ allocation and onlining.
> >> Also, I've just move the allocation to alloc_mem_cgroup_per_node_info() like it was
> >> suggested in comments to v1...
> > 
> > Yeah, but (ab)using mem_cgroup_idr for iterating over all allocated
> > memory cgroups looks rather dubious to me...
> 
> But we have to iterate over all allocated memory cgroups in any way,
> as all of them must have expanded maps. What is the problem?
> It's rather simple method, and it faster then for_each_mem_cgroup()
> cycle, since it does not have to play with get and put of refcounters.

I don't like this, because mem_cgroup_idr was initially introduced to
avoid depletion of css ids by offline cgroups. We could fix that problem
by extending swap_cgroup to UINT_MAX, in which case mem_cgroup_idr
wouldn't be needed at all. Reusing mem_cgroup_idr for iterating over
allocated cgroups deprives us of the ability to reconsider that design
decision in future, neither does it look natural IMO. Besides, in order
to use mem_cgroup_idr for your purpose, you have to reshuffle the code
of mem_cgroup_css_alloc in a rather contrived way IMO.

I agree that allocating parts of struct mem_cgroup in css_online may
look dubious, but IMHO it's better than inventing a new way to iterate
over cgroups instead of using the iterator provided by cgroup core.
May be, you should ask Tejun which way he thinks is better.

Thanks,
Vladimir